I. Field of the Invention.
This invention relates generally to a control circuit for use with the windshield wiper motor on a vehicle, and more particularly to a circuit which senses the presence of moisture droplets on the windshield to activate the wiper motor and adjust its speed in relation to the intensity of the precipitation encountered.
II. Discussion of the Prior Art
Most of today's motor vehicles embodying windshield wipers utilize a wiper system in which the wiper motor is controlled to operate intermittently or at slow or fast rates depending upon the setting of a manual switch lever by the driver. Such wiper control systems also now often include a variable delay feature, whereby the time period between successive strokes, when operating in the intermittent mode, can be adjusted.
In our earlier U.S. Pat. No. 4,620,141, there is disclosed an electronic control circuit for a windshield wiper motor which includes a windshield mounted sensor module incorporating a plurality of radiant energy sources, such as LEDs, which are pulsed on and off in accordance with a pre-established duty cycle and a further plurality of radiant energy sensors, e.g., photo-transistors, which are oriented relative to light transmitting channels such that they intersect at the outside surface of the windshield. The plural sensors are connected in a bridge configuration, and when water droplets impinge on the windshield, the light from the sources is refracted to unbalance the bridge. A synchronous demodulator circuit receives the output from the bridge and the switching of the synchronous demodulator is in phase with the pulsed energization of the radiant energy sources in the sensor module.
The output from the synchronous demodulator is then presented to an AGC amplifier configured to compare that signal to a predetermined reference. When the sensor bridge becomes unbalanced due to moisture on the windshield, the signal is applied to a window comparator which produces a binary signal irrespective of the direction of change. The binary signal is then integrated and applied to a voltage-controlled oscillator that is operatively coupled in circuit with the windshield wiper relay circuits to drive the wipers at a rate which varies as a function of the level of perception being encountered.
While the apparatus of our earlier patent was fully operative to control the wiping speed in accordance with precipitation levels, it was found to suffer from the drawback that sudden or pulsatile changes in ambient light striking the sensor module would frequently cause false triggering and activation of the wipers even when it was not raining. For example, when driving down a highway with shadows of telephone poles or the like sweeping across the windshield, the circuit in our earlier patent would occasionally be "fooled", resulting in false-triggering of the wiper motor. To be commercially practical, all optical rain-sensing wiper controls must be capable of dealing with the noise caused by shifts in ambient light. With prior art rain-responsive wipers, the ambient light problem has been a difficult one to deal with in that the sensing circuitry employed has been unable to distinguish between the infrared energy modulation caused by rain impinging on the windshield and changing infrared energy coming from the sun being modulated by shadows from bridges, telephone poles and the like when the vehicle is driving down the highway. In the aforereferenced McCumber patent, an attempt was made to reject ambient disturbances by canceling out common-mode noise. That system also incorporated filtering to further reduce the effects of ambient light on the rain sensor. The systems described in the prior art patents Larson et al. U.S. Pat. No. 4,859,867 and Fujii et al. U.S. Pat. No. 4,867,561, each of which relates to a rain-responsive windshield wiper control, attempt to make the control immune to ambient light by using a separate optical light sensor and then using its output to shift the threshold level at which a rain present determination is made. That is to say, a separate optical rain sensor and a separate optical light sensor are used to feed their respective outputs into a digital comparator. Each of these approaches, however, suffers from the problem that small amounts of rain cannot be detected in the presence of large amounts of light. The digital comparison merely indicates that there is more rain than light. It is important to a commercially viable rain responsive windshield wiper control that it respond to light rains as well as down-pours, even in bright sunlight.
The systems described in the aforereferenced Larson and Fujii patents require a separate sensor for the rejection of ambient light which, of course, adds to the cost of the system. More importantly, however, the effective ambient light may not be the same on the light sensor element as it is on the rain sensor element. Thus, the approach described therein has severe limitations on how accurately it can measure the ambient light disturbances.
Another problem with prior art systems involves the operation when rain may be falling in a pattern that misses the area under view of the sensor, yet obscures the windshield. All prior art systems which utilize an active sense area that is small compared to the size of the windshield suffer from this problem, and examples may be found in the aforementioned Larson, Fujii as well as Fukatsu et al. U.S. Pat. No. 4,595,866, Gibson U.S. Pat. No. 4,463,294, Shiraishi U.S. Pat. No. 4,527,105, Watanabe et al. U.S. Pat. No. 4,481,450, Noack U.S. Pat. No. 4,355,271 and others. In the present invention, the water that missed the sensor is gathered up by the windshield wipers, passed over the sensor, and detected. This intentional retriggering algorithm thus effectively utilizes the entire swept area of the windshield to detect water, obviating this problem.
A further shortcoming of the prior art systems based upon optical technology is that they will tend to lose sensitivity over the life of the vehicle, as the windshield surface becomes pitted and the optical quality of the system degrades. Further, the system will perform differently for windshields of different transmittance. Most prior art arrangements simply permit the unit's performance to degrade, as in Schierbeek et al. U.S. Pat. No. 4,916,374. The aforementioned McCumber patent teaches that one may use an automatic gain control (AGC) which will adjust the sensitivity of the system to compensate for these variations. The problem with the AGC approach is that as the system increases its gain to achieve proper sensitivity, the device will amplify ambient light disturbances and other noise along with the desired signal. Thus, the signal-to-noise ratio of the system declines. The present invention teaches that one may detect the strength of the signal coming back to the photodetectors and appropriately change the intensity of the photoemitters, allowing the system to function as a constant signal-to-noise ratio device.